Re: [BUG] using smp_processor_id() in preemptible

[Michal Hocko <mhocko@suse.cz>]

On Sun 16-11-08 08:10:59, Ingo Molnar wrote:
> 
> * Luis Henriques <henrix@sapo.pt> wrote:
> 
> > Hi Ingo,
> > 
> > while running latencytop, got the following:
> > 
> > [  775.663239] BUG: using smp_processor_id() in preemptible [00000000] code: latencytop/6585
> > [  775.663303] caller is native_sched_clock+0x3a/0x80
> > [  775.663314] Pid: 6585, comm: latencytop Tainted: G        W 2.6.28-rc4-00355-g9c7c354 #1
> > [  775.663322] Call Trace:
> > [  775.663343]  [<ffffffff803a94e4>] debug_smp_processor_id+0xe4/0xf0
> > [  775.663356]  [<ffffffff80213f7a>] native_sched_clock+0x3a/0x80
> > [  775.663368]  [<ffffffff80213e19>] sched_clock+0x9/0x10
> > [  775.663381]  [<ffffffff8024550d>] proc_sched_show_task+0x8bd/0x10e0
> > [  775.663395]  [<ffffffff8034466e>] sched_show+0x3e/0x80
> > [  775.663408]  [<ffffffff8031039b>] seq_read+0xdb/0x350
> > [  775.663421]  [<ffffffff80368776>] ? security_file_permission+0x16/0x20
> > [  775.663435]  [<ffffffff802f4198>] vfs_read+0xc8/0x170
> > [  775.663447]  [<ffffffff802f4335>] sys_read+0x55/0x90
> > [  775.663460]  [<ffffffff8020c67a>] system_call_fastpath+0x16/0x1b
> > ...
> > 
> > I am running 2.6.28-rc4-00355-g9c7c354 and, after taking a look at 
> > the code, it looks like the problem is in commit 
> > 7cbaef9c83e58bbd4bdd534b09052b6c5ec457d5, where native_sched_clock 
> > is modified to use __cycles_2_ns instead of cycles_2_ns.
> 
> indeed. Does the fix below work for you?

Yes, it fixes problem on my end... (reported at
http://www.gossamer-threads.com/lists/linux/kernel/997205).

> 
> Thanks,
> 
> 	Ingo
> 
> -------------->
> From 29d7b90c15035741d15421b36000509212b3e135 Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@elte.hu>
> Date: Sun, 16 Nov 2008 08:07:15 +0100
> Subject: [PATCH] sched: fix kernel warning on /proc/sched_debug access
> 
> Luis Henriques reported that with CONFIG_PREEMPT=y + CONFIG_PREEMPT_DEBUG=y +
> CONFIG_SCHED_DEBUG=y + CONFIG_LATENCYTOP=y enabled, the following warning
> triggers when using latencytop:
> 
> > [  775.663239] BUG: using smp_processor_id() in preemptible [00000000] code: latencytop/6585
> > [  775.663303] caller is native_sched_clock+0x3a/0x80
> > [  775.663314] Pid: 6585, comm: latencytop Tainted: G        W 2.6.28-rc4-00355-g9c7c354 #1
> > [  775.663322] Call Trace:
> > [  775.663343]  [<ffffffff803a94e4>] debug_smp_processor_id+0xe4/0xf0
> > [  775.663356]  [<ffffffff80213f7a>] native_sched_clock+0x3a/0x80
> > [  775.663368]  [<ffffffff80213e19>] sched_clock+0x9/0x10
> > [  775.663381]  [<ffffffff8024550d>] proc_sched_show_task+0x8bd/0x10e0
> > [  775.663395]  [<ffffffff8034466e>] sched_show+0x3e/0x80
> > [  775.663408]  [<ffffffff8031039b>] seq_read+0xdb/0x350
> > [  775.663421]  [<ffffffff80368776>] ? security_file_permission+0x16/0x20
> > [  775.663435]  [<ffffffff802f4198>] vfs_read+0xc8/0x170
> > [  775.663447]  [<ffffffff802f4335>] sys_read+0x55/0x90
> > [  775.663460]  [<ffffffff8020c67a>] system_call_fastpath+0x16/0x1b
> > ...
> 
> This breakage was caused by me via:
> 
>   7cbaef9: sched: optimize sched_clock() a bit
> 
> Change the calls to cpu_clock().
> 
> Reported-by: Luis Henriques <henrix@sapo.pt>

Tested-by: Michal Hocko <mhocko@suse.cz>

> ---
>  kernel/sched_debug.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
> index 48ecc51..26ed8e3 100644
> --- a/kernel/sched_debug.c
> +++ b/kernel/sched_debug.c
> @@ -423,10 +423,11 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
>  #undef __P
>  
>  	{
> +		unsigned int this_cpu = raw_smp_processor_id();
>  		u64 t0, t1;
>  
> -		t0 = sched_clock();
> -		t1 = sched_clock();
> +		t0 = cpu_clock(this_cpu);
> +		t1 = cpu_clock(this_cpu);
>  		SEQ_printf(m, "%-35s:%21Ld\n",
>  			   "clock-delta", (long long)(t1-t0));
>  	}
> 
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-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic